The present invention relates to a position detecting device that detects a position pointed by a position pointer using electromagnetic coupling and that is used for an input device of, for example, a computer aided design (CAD) terminal, a computer, a personal digital assistant (PDA), a cell phone, and a personal handyphone system (PHS).
Position detection devices have been used which receive an electromagnetic coupling signal from a position pointer for pointing a position so as to detect a position pointed by the position pointer and which are used for an input device of, for example, a CAD terminal, a computer, a PDA, and a cell phone.
FIG. 6 is a timing diagram of a known position detecting device in which sensor coils are arranged in an XY matrix. As shown in FIG. 6, if the known position detecting device has already recognized a position pointed by a position pointer, a predetermined number of X-axis direction sensor coils and a predetermined number of Y-axis direction sensor coils arranged in the vicinity of a position pointed by the position pointer (sensor coils 1 to 6 in FIG. 6) are alternatively scanned in a direction in which the sensors are arranged (i.e., a direction from the sensor coil 1 to the sensor coil 6) so that each of the sensor coils and the position pointer communicate signals. Thus, the position detecting device detects the position pointing signals from the position pointer and computes the position pointed by the position pointer using, for example, parabolic approximation.
At that time, as shown in FIG. 7, each sensor coil transmits a detecting signal to the position pointer and receives a position pointing signal from the position pointer several times (four times in an example shown in FIG. 7) to compute the position pointed by the position pointer by using the sum or average value of signal levels obtained from the receiving operations.
However, a position detecting device used at a position very close to the body of an electronic apparatus, such as a computer and a PDA, has a disadvantage in that a significant error occurs when detecting a position pointed by a position pointer due to electromagnetic noise which is generated by the body of the electronic apparatus and which influences a signal received from the position pointer using electromagnetic coupling.
For example, in a position detecting device used as an input device of an electronic apparatus such as a PDA or a cell phone, noise in a driving signal of a liquid crystal display (LCD) incorporated in the body of the electronic apparatus (for example, noise in a display driving signal or in a backlight inverter) disadvantageously causes a significant detection error.
In particular, when, as shown in FIG. 6, a scanning frequency of the sensor coils in the position detecting device is close to a frequency of noise from the body of the electronic apparatus, that is, when the difference between both frequencies is small, low-frequency jitter is generated, since the sensor coils are scanned in the order in which the sensor coils are arranged. Thus, it is difficult to eliminate the influence of the noise. Consequently, the detected position may vary and a significant error may occur. Alternatively, the level of a signal detected by a specific sensor coil may be high, and therefore, the signal is detected as if the position of the position pointer is displaced. Thus, a detection error becomes large, which is a problem.
FIG. 8 illustrates a frequency characteristic of jitter when an electromagnetic coupling signal received by a position pointing device from a position pointer is affected by noise generated by an LCD in an electronic apparatus. As shown in FIG. 8, when the unit interval of a sensor-coil scanning frequency is a reciprocal number 1/Ts of scanning time Ts for scanning two coils of the position detecting device (or one coil depending on a computing method of the coordinates of the position detecting device), a large low-frequency jitter occurs for noise at positions of a frequency distant from a sensor-coil scanning frequency f0 by ±0.5 frequencies, that is, at positions of f0±0.5/Ts. Accordingly, a large detection error disadvantageously occurs due to the noise.